James Gurney

This daily weblog by Dinotopia creator James Gurney is for illustrators, plein-air painters, sketchers, comic artists, animators, art students, and writers. You'll find practical studio tips, insights into the making of the Dinotopia books, and first-hand reports from art schools and museums.

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All images and text are copyright 2015 James Gurney and/or their respective owners. Dinotopia is a registered trademark of James Gurney. For use of text or images in traditional print media or for any commercial licensing rights, please email me for permission.

However, you can quote images or text without asking permission on your educational or non-commercial blog, website, or Facebook page as long as you give me credit and provide a link back. Students and teachers can also quote images or text for their non-commercial school activity. It's also OK to do an artistic copy of my paintings as a study exercise without asking permission.

Friday, December 14, 2007

Here’s a study from life of a giant albino bullfrog from an aquarium. The creature was the size of a plucked chicken, and about the same color. He held still for twenty minutes while I did this study.

I drew him in pencil on gray mat board, and then laid a milky wash of opaque watercolor over his whole body, saving the brightest whites for the accents and highlights. When the overall light wash was dry, I added the dark accents in pencil. These include the pupil of the eye and the places where forms push together in the folds and wrinkles.

Wherever two forms touch each other, or a form touches a floor, a dark line or accent results. You can see the effect by pressing your fingers together and looking at the little dark line where they touch. Not much light makes it to that point of contact. You’ll also notice it gets darker in the inside corner of a room where the walls meet.

Computer lighting programs don’t create this dark accent automatically. Until recently it had to be added by hand. But software pioneers have recently made lighting tools that can anticipate when the light will be occluded and such an accent will appear.

As a traditional oil painter, I'm fascinated by such new terminology and visual analysis developed my brother artists in the CG arena. I wonder if one of you who is familiar with 3-D CG lighting might be willing to comment on the challenges presented by occlusion shadows.

I do a bit of work with 3d programs, and the fundamental thing one has to learn is that light doesn't bounce, nor is it stopped particularly evenly. Since ambient lighting in the real world is mostly caused by light bouncing from surfaces, or refracting through matter, rather than directly from a light source, this means that to create the effect of one light source in reality, you must use many, many lights in a 3d program (not sure about the super high end gear that film studios use though). Occluding light is very hard to do, as most of the algorithms that calculate exactly how to place light within a scene cannot manage that degree of accuracy without you fiddling with it for hours and spending hours more rendering.

However, I'm very much an amateur, not a professional in the field, and I'm sure the masters of the art have quite a few ways of accurately getting proper occlusion. It's not impossible, but very difficult.

I think maybe the biggest challenge for CG artists has just been waiting for the computer to calculate it. But that's getting so much better now. Modern software makes such shadowing so easy it can feel like cheating. For artists it can almost be as easy as clicking a button to enable such shadowing.

In traditional CG a single light simulating the sun would not "bounce" off the CG ground and reflect soft shadowed light up under the eaves of a CG house. Nor would the blue CG sky cast any soft shadowed blue light into shaded areas.

You could fake the effect with multiple lights (as someone commented above), but with modern rendering technology you can skip that and actually render without a single traditional CG light at all. The below image was rendered with all illumination coming from a single High Dynamic Range Image photograph which captures a wide contrast range of light from a real world location. http://www.joshsukov.com/HDRI_lighting.jpg

HDRI photos can cast light into your rendering from 360 degrees in every direction including different colors that were captured such as blue sky or green grass. One downside to it is that artist control is somewhat limited by the captured imagery that is generating light. However, there are other lighting approaches with more control which still utilize occlusion shadowing.

I'm also a 3D artist who uses both occlusion shadows and HDRI lighting on a daily basis, and I thought I'd provide a quick sample as well.

I'm making a series of commericals featuring talking snails, and when the animation is finished, the final result is output into many different image layers which are then combined to create the final image. Occlusion shadows is only one of these layers.

Here's an image of the snail sitting in the garden. This is the most basic layer, called the 'beauty pass'. The snail is lit using HDRI lighting as Joshua explained for the bounce and reflected light, and is supplemented by traditional 3D lights as thereisnosaurus explained. Then the snail is placed over a photographed background. This is before any other layers are applied:

http://www.whiteiron.tv/bin/abilawchuk\Snails_BEAUTY.jpg

This is what the Occlusion pass looks like on its own, demonstrating the effect that James described:

http://www.whiteiron.tv/bin/abilawchuk\Snails_OCCONLY.jpg

Here are both layers combined together, but it's still not done:

http://www.whiteiron.tv/bin/abilawchuk\Snails_AMBOCC.jpg

Here's both layers with an additional layer of shadows cast from my supplimental 3D lights:

http://www.whiteiron.tv/bin/abilawchuk\Snails_SHADOW.jpg

That's near to completion, but there are many other layers that would go into the final as well. Specular highlight and reflection layers, for example.

Whew! Hope that helps illustrate and tie together what James, thereisnosaurus, and Joshua said! Hopefully someone will read this, too!

Great explanation, Thereisnosaurus, Joshua, and AaronZoom. I'm sure other painters like me appreciate the time all of you put into telling--and showing--how you do it in 3-D digital. It's fascinating to click back and forth on the snail images and see how the image goes together.

This is one of the first posts that I have read on your blog and it's great! I'm definitely putting your link in my "daily" list. :)

As for the subject, I'm an 3D artist that works in the video game industry as a lighter and pretty much what has been said above is spot on.

The only thing I can add is that now real-time lighting for games is adding a new twist to how to create ambient occlusion shadows.

Instead of rendering and compositing a frame for final output like in film, the programmers in the game industry are starting to develop methods to create ambient occlusion in real-time in game worlds.

What happens is that any polygon that meets another polygon (or gets in close proximity to it) they both receive a slight darker shadow in a post process (after everything is rendered in the scene). Examples would be a character stands near a wall it or a basket of fruit falls the ground.

You can tweak the effect so that you can have darker or larger gradients creeping up on the objects. Pretty fun to see in real-time in a game. :)

The giant albino frogs can not be compared with other species of frogs. Definitely are my favorite. I've heard they possess a similar effect such as the Viagra Online, they secrete a kind of liquid that makes men improve their sexual desire.